Respect NO_COLOR environment variable

[ima1zumi]

I want the completion dialog to have no color if the NO_COLOR environment variable is present.

ref ruby/irb#327

[avsej]

A temporary solution would be this snippet for $HOME/.irbrc:

require "reline/face"

if ENV.key?("NO_COLOR")
  Reline::Face.config(:completion_dialog) do |conf|
    conf.define :default, foreground: :white, background: :black
    conf.define :enhanced, foreground: :black, background: :white
    conf.define :scrollbar, foreground: :white, background: :black
  end
end

RDoc still in color though, but at least what related to reline works as expected.